Known in the past has been an exhaust purification system which provides an air-fuel ratio sensor at an upstream side of an exhaust flow direction of an exhaust purification catalyst which is provided in an exhaust passage of an internal combustion engine and provides an oxygen sensor at a downstream side in the exhaust flow direction of the exhaust purification catalyst. In such an exhaust purification system, for example, the output of the upstream side air-fuel ratio sensor is used as the basis for feedback control of the amount of fuel which is supplied to the internal combustion engine so that the air-fuel ratio of the exhaust gas which flows into exhaust purification catalyst becomes the target air-fuel ratio (main feedback control) and the output of the downstream side oxygen sensor is used as the basis for feedback control of the target air-fuel ratio (sub feedback control).
In this regard, an oxygen sensor etc. which is used in such an internal combustion engine sometimes becomes abnormal due to cracking of the element which forms the sensor. In such a case, the sensor not longer generates a suitable output in accordance with the air-fuel ratio of the flowing exhaust gas. Therefore, it is known to provide an abnormality diagnosis system which diagnoses such an abnormality of a sensor.
In such an abnormality diagnosis system, for example, it is known to diagnose abnormality as follows: That is, when the output air-fuel ratio of the downstream side oxygen sensor becomes an air-fuel ratio leaner than the stoichiometric air-fuel ratio (below, also referred to as a “lean air-fuel ratio”), the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst is made an air-fuel ratio richer than the stoichiometric air-fuel ratio (below, also referred to as a “rich air-fuel ratio”). After this, when the output air-fuel ratio of the downstream side oxygen sensor does not invert to the rich air-fuel ratio even if the stored amount of oxygen of the exhaust purification catalyst becomes zero, the oxygen sensor is diagnosed to have become abnormal due to a cracked element (for example, PLT 1). According to PLT 1, it is made possible to quickly and precisely detect abnormality of the oxygen sensor.